Diverse applications ranging from decorative coatings for jewelry and automotive parts to functional films in microelectronics utilize thin film technology. Thin film processes include vacuum deposition (evaporation, sputtering, chemical vapor deposition), spin coating and plating. Vacuum and spin coating processes require the use of photolithographic techniques to create the desired pattern. These processes can be labor intensive and not very economical for high volume coating processes.
Plating processes are more economical for metallizing large volumes of parts. Plating processes can be divided into two distinct types: electrolytic and electroless plating. Electrolytic plating is a standard process used to deposit a uniform metal thickness over electrically connected features. This process requires that the pattern to be plated is connected to an external power source by electrical leads. Part specific tooling is usually required to made reliable electrical connections to each part. Excess metallization is used to ensure all features are electrically connected and that uniform potential exists across the part during electrolytic plating. These excess metal features must be removed in a separate process. In addition, deposition of excess metal can lead to overplating and shorting of the electrical circuit. Terminators are often left that produce undesirable high frequency electrical characteristics. Therefore, electrolytic plating of electrically isolated regions is labor intensive and costly.
The electroless plating process deposits a uniform metal thickness over catalyzed features without the application of an external power source. This process takes advantage of thermodynamically feasible redox reactions between the catalyzed surface and chemical constituents in the electroless plating bath. A true autocatalytic electroless bath continues to build up a metal layer on the catalytic feature even after the initial surface has been completely covered by the metal that is being plated.
Electroless plating appears to be the most effective method for large scale, selective metallization; however, there are problems associated with commercial applications of some electroless plating solutions. Electroless bath chemistries are thermodynamically unstable and require very specific and precise formulations in order to maintain stability throughout numerous plating runs. Electroless baths also require careful maintenance because very low contamination levels can destabilize the bath. The baths are easily contaminated by the large volume of parts that are immersed into the plating solution. The costs associated with metal recovery, waste treatment, waste disposal; and maintenance costs of the large plating baths deter the use of electroless plating in many applications.
Another issue that arises is the compatibility of the bath chemistry with the material to be plated. For instance, commercially used autocatalytic electroless gold plating baths have a high pH to ensure stability of the reducing agent. These formulations can be corrosive to the material being plated.
In addition, the high pH electroless plating solutions destroy resist coatings used in the process. Masking techniques combined with successive runs in a plating bath are often used to achieve variation of metal thicknesses on the same substrate or to prevent plating on various areas of the substrate. The high pH electroless plating solutions destroy the resists often used in these masking applications.
Further, the high pH electroless plating solutions are cyanide-based. The health risks associated with such baths make them extremely undesirable. It would be advantageous to reduce or eliminate the cyanide levels in the electroless bath solution.
The problems associated with electrolessly gold plating selective areas of an aluminum nitride (AlN) substrate for microelectronic applications illustrate the limitations of the current electroless plating technology. AlN is a potential replacement for alumina in small, high power electronic devices. However, the commercially used electroless plating solution etches AlN because of its high pH. This corrosion rate is accelerated at the elevated temperatures used for plating operations. The surface properties of AlN are significantly altered during plating which not only damages the prior processing steps but also complicates further processing of the ceramic package. Any defectively plated parts add significantly to the final cost.
One approach to obtain an economical selective gold plating process compatible with AlN is to protect the exposed aluminum nitride surface from the corrosive plating solution. U.S. Pat. No. 5,306,389 discloses a method of protecting partially metallized aluminum nitride substrates during electroless plating in a gold electroless plating solution, by converting the exposed aluminum nitride to alumina through a surface oxidation treatment. This is counterproductive; however, as it is desirable to limit the presence of alumina on the AlN substrate since alumina has a lower thermal conductivity than AlN.
It is therefore desirable to develop a metallization process which avoids degradative reaction of the AlN surface.
Okinaka et al., Plating, September 1970, p. 914 and U.S. Pat. No. 3,700,469, disclose a typical autocatalytic electroless gold plating solution containing a gold-cyanide complex (KAu(CN).sub.2) that is reduced by a borane reducing agent, dimethylamine borane (DMAB). Such a bath has a pH around 14, a gold concentration of about 4 g/L and a plating temperature of about 82.degree. C.
Mathe et al., Metals Finishing, January 1992, p. 34, disclose additives to an electroless plating bath (and their functions). Additive include stabilizers that inhibit the solution decomposition by masking active nuclei, buffers which maintain the proper pH, organic chelating agents that act as a buffer and/or prevent rapid decomposition.
Sullivan et al., J. Electrochem. Soc., Vol. 142, No. 7, July 1995, p. 2250, describes a non-cyanide, non-alkaline electroless gold plating bath in which sodium gold(I) thiosulfate (Na.sub.2 Au(S.sub.2 O.sub.3)) is used as the gold complex and sodium L-ascorbic acid is used as the reducing agent. The bath has a pH of 6.4, deposition rates of 1 micron/hour and a plating temperature of 30.degree. C. The non-toxicity and low pH of this bath makes it an attractive alternative to current cyanide alkaline baths, especially for AlN substrates. However, these baths are not as stable or reliable (note 30.degree. C. deposition temperature) as the high pH cyanide baths currently used in manufacturing. U.S. Pat. No. 5,470,381 identifies a stabilizing agent which prevents rapid decomposition of the lower pH electroless gold plating solutions for gold concentrations of approximately 2 g/L; however, such gold concentrations are undesirably dilute.
Alternate selective metallization techniques that have been found in the literature include a technique which incorporates meltable salts into an ink that is printed onto a substrate using ink jet printing (Ishwar Ramchand Manshani, Japanese Patent S54-4247). This technique results in a flash deposit of metal on the substrate surface. Flash deposit occurs because of the galvanic displacement of the less noble metal substrate by the more noble metal in the ink. Therefore, this technique is not an autocatalytic plating process so the resulting metal deposit is limited to a very thin coating. The ink jet printing technique also is discussed in U.S. Pat. Nos. 3,465,350 and 3,465,351.
It is the object of the present invention to overcome the limitations of prior art electroless plating baths and operations described herein above.
It is a further object of the invention to eliminate or minimize etching and other surface defects associated with conventional high pH electroless plating operations and baths.
It is a particular object of the invention to provide an electroless bath and plating process for the plating of gold on AlN substrates.
It is a further object of the invention to provide an electroless bath and plating process which allows selective variation of metal thickness on the same substrate.
It is yet a further object of the invention to eliminate or minimize electroless plating bath maintenance and stability concerns.
It is a further object of the invention to provide a rework procedure for defectively plated substrates.
It is a further object of the invention to optimize the usage of metal in the deposition process.
It is a further object of the invention to reduce the volume of waste generated in the electroless plating process.
It is a further object of the invention to reduce the overall cost of the plating process.
It is a yet a further object of the invention to control deposition of the plated metal to minimize overplating problem encountered in conventional electroless plating operations.